Combined cycle power plants are well known in the art. A combined cycle power plant includes both a gas turbine-based topping cycle and a steam turbine or a steam ranking bottoming cycle that is driven by heat in the exhaust of the gas turbine. During startup of a combined cycle power plant from cold start conditions, the gas turbine portion of the plant necessarily must be started before the steam turbine portion. The term cold start is a relative term but is used herein to refer generally to conditions where the plant has not been operated for an extended time period, such as 48 hours, and where the boiler is not pressurized. During startup of a gas turbine having a single shaft-constant speed arrangement, there is a relatively rapid increase in the flow rate of the exhaust from the gas turbine as it accelerates to operating speed. Thereafter, the exhaust gas flow rate remains relatively constant except for the effect of compressor inlet guide vane modulation. After the gas turbine reaches operating speed, the temperature of the exhaust gas gradually increases as the firing temperature of the gas turbine is increased up to the level required to produce the desired power output. However, the rate of increase in load and temperature of the gas turbine exhaust is constrained by thermal transient stress limits in the components of the steam turbine and the balance of plant, including the heat recovery steam generator (HRSG) that is exposed to the hot exhaust gas stream. During startup, the startup temperature of the gas turbine exhaust is regulated to gradually heat and pressurize the HRSG. In a typical combined cycle plant, the gas turbine may be initially limited to about 20–30% rated power in order to maintain the exhaust at a sufficiently low temperature to maintain stresses within acceptable levels in the cold HRSG.
The necessity to gradually heat a combined cycle power plant during startup reduces the overall efficiency of the plant and reduces the plant's ability to respond to rapidly changing power requirements. Furthermore, the operation of the gas turbine portion of the plant at less than full rated load may result in a level of gaseous emissions that exceeds regulatory or Original Equipment Manufacturers base load contractual requirements. In particular, it is known that the level of carbon monoxide (CO) produced in a gas turbine engine will increase as the firing temperature is decreased during part-load operation. Operation of the gas turbine portion of a combined cycle power plant at 20–50% rated load during the startup phase will often place the plant outside of emissions compliance limits. Not only does such operation have an undesirable impact on the local environment, but it may also have a negative financial impact on the owner or operator of the plant, since a plant revenue stream may be adversely impacted by operation outside of regulatory compliance limits. Accordingly, there is a strong incentive to reduce the startup time for a combined cycle power plant and to reduce the operation of the plant at non-compliance emissions points.